Electronic or manual fingerprinting for visa FBI background checks?

Can anyone provide any insight if it's worth doing the $50 USPS walk-in electronically-filed fingerprinting vs. $10 police station fingerprinting? It would be $100 vs. $20 since we have two kids that need them (yes, kids have to do them)…

I heard electronic ones are processed more quickly–but we aren't really in a rush. My primary concern is issues with the fingerprints not being adequate (especially since these are tiny kid fingers) and returned. I feel like issues would be more likely at a police station if done manually with a physical card (and of course, we'd likely not find out for weeks, and would have to do it again)? Seems like the USPS wouldn't accept bad prints? But would not mind saving $80.